Composition of matter and process



Reissued June 30, 1942 COMPOSITION OF MATTER AND PROCESS OF PREPARINGTHE SAME Joseph E. Plumstead,

by mesne assignmen Paper Company, New York, N. Y.-

of Delaware No Drawing. Original No.

Wilmington, lleL, assignor ts, to West Virginia Pulp a a corporation2,186,040 dated January 9, 1940, Serial No. 746,266, September 29,

1934, which is a July 11, 1933.

division of Serial No. 679,993, Application for reissue November 16,1940, Serial No. 365,992. Renewed December 27, 1938 18 Claims.

This invention relates to a loading material, filler, or pigment, and tothe manufacture thereof, and more particularly to methods of treatingfinely divided solid materials to improve their characteristics asfillers, pigments, or loading materials for use in various materials.

An object of the present invention is to provide pigment or fillermaterial having highly desirable characteristics. Another object is toprovide a method of treating pigment or filler material by whichparticularly its opacity is improved.

By the process of the present invention, a material which is ordinarilyemployed as a pigment or filler, or in the manufacture thereof, may beconverted into a highly improved product by treating a suspension ofparticles of the material in a. fluid with a reagent which will effectselective removal of portions of the particles whereby the particles arerendered spongy. vesicular and amorphous in character and whereby theregularity and angularity of shape of the particles are destroyed. Thiscauses the particles to have a greater liquid absorptivity and what ismore important a greater hiding power, greater opacity, increasedsoftness and smoother surface characteristics when used as fillers, orpigments.

The percentage of retention of such treated particles is increased dueto the increased clinging characteristics of the honeycomb surfacestructure. A pigment substance, and a chemical reagent which affects thesurface of pigment particles by removing matter from their surfaces andin certain instances by adding matter to their surfaces, as for example,by precipitation of substances thereon, are employed in the presentprocess. The qualities of the final product are greatly influenced bythe physical shape and chemical reactivity of the initial pigmentmaterial.

For example, a suspension of calcium-magnesium hydroxides prepared byslaking burned dolomitic lime will include finely comminuted particlesin which the magnesium portion is more readily dissolved by commonacidic materials than the calcium portion. The result of this method incomparison to treating a finely comminuted dolomitic limestone is thatthe resultant vesicular pigment whether it be in the form of carbonate,sulphite, sulphate, or a mixture of these, with the hydroxides of theaforementioned slaked material will be a better product when preparedfrom the latter material than when prepared from the ground limestone.The product prepared from ground limestone may, however, be made verynearly equal in quality to that prepared from slaked lime if the productobtained pension to form wholly or partly soluble after treatment withthe reagent is finally wet ground for instance.

The present invention includes the treatment with acidic material ofalkaline earth carbonate, hydroxide, sulphate or sulphite particles insusacid salts. After the formation of such salts there may be atreatment of the acid salt solution either after separation of thesolution from the suspended unreacted portions of the particles or whilethe latter particles are still mixed with the solution. In order towholly or partially neutralize the acid salt solution and so that thewhole or part of the base portion of the acid salt will be precipitatedas additional pigment material.

The above process may be varied to form at least three types ofpigments. One is the pigment residue obtained by the treatment withreagents; an amorphous type having a. honeycomb structure and beingvesicular. This may or may not be ground and may or may not be dried.Another type includes a combination or the pigment residue of honeycombstructure with a precipitated pigment which may be wholly amorphous oramorphous and crystalline. Still another type includes a precipitatedpigment which may be wholly crystalline or wholly amorphous, or acombination of the two.

In the manufacture of cooking acid for use in digesting pulp in theso-called sulphite process, a sulphurous acid solution of calciumbisulphite or calcium and magnesium bisulphites has been used for manyyears. I have found it to be particularly advantageous from an economicstandpoint to produce my pigment product by the process of the presentinvention while preparing the cooking acid. Accordingly, the cookingacid is prepared from lime-bearing substances, such as finely dividedcalcium limestone, or dolomitic limestone, or slaked lime from either ofthese limestones, and sulphur dioxide in the presence of water bycarrying out the usual acid making operation to partial completion only.In this instance, suspended particles of the lime and/or magnesiummaterial are removed from the acid either by filtering, decanting, orotherwise, before being completely reacted upon. The resultant solutionor filtrate is employed in the usual sulphite pulp digesting process,and the filter cake may be washed or treated with milk of lime andemployed as a filler or pigment.

A method of preparing my product may be outlined in a general way asfollows:

(1) Burn dolomitic limestone.

(2) Slake dolomitic lime.

. stances.

Another method is one which may be used with calcium lime by includingsteps one and two as above and proceeding as follows:

(3) Treat the hydroxide with suflicient acidic material to convert thehydroxide into the salt of the acid.

(4) Add enough more of the acid to partially dissolve the salt and formthe amorphous compound in aqueous suspension.

(5) Add a basic metallic or alkaline salt such as carbonate or hydroxideto react with the dissolved acid salt formed in 4 so that the normalsalt will be thrown down as additional pigment material.

In carrying out the process with finely ground limestone material thesteps are as follows:

(1) Grind the limestone material as finely as may be desired.

(2) Treat the ground limestone material with an acidic selective solventsuch as carbonic acid solution, sulphurous acid, sulphurous acidsolution of bisulphite or sulphuric acid to partial, selectivedissolving or a corroded state.

(3) Treat the resultant material of step 2 with a basic metallic oralkaline salt such as carbonate or hydroxide or filter ofl dissolvedsalt for recovery or wasting.

(4) Grind and dry or dry and grind as may be desired.

Carbonate particles treated in accordance with the present process havesubstantially the same or a greater degree of opacity when compared withparticles of domestic clays or China clays or kaolins. of similar colorand of approximately the same size. The treated carbonate particles haveno definite crystalline formation, but on the contrary are sponge-like,pitted bodies having corroded edges which are visible when examinedunder a high powered microscope. The elimination of sharp edges fromcrystalline particles renders the product highly desirable for manypurposes.

The spongy character or my product is indicated by its high waterholding power even when subjected 'to reduced pressures belowatmospheric. Ordinary precipitated or crystalline calcium carbonate whenmatted to a one-half inch cake on the plate of a vacuum press andsubjected to 18 inches of vacuum for a. period of three minutes willtest less than 40% moisture. Calcium carbonate particles treated by myprocess will test over 40% 0! moisture when sub- .iected to the sameconditions. a

In the manufacture of sodium hydroxide and precipitated calciumcarbonate from sodium carbonate and calcium hydroxide according to thereaction,

certain amounts of calcium hydroxide, caustic soda and sodium carbonateare left mixed with the predominating calcium carbonate. It is wellknown that the above reaction is reversible according to theconcentrations used and that the formation or calcium carbonate fromthis reaction is never complete. Therefore, the calcium carbonateresidue, even though thoroughly washed, is alkaline and contains varyingquantities of calcium hydroxide which is preferably neutralized torender the residue satisfactory for commercial use as calcium carbonate,or precipitated chalk. I have discovered that if this surplus ofalkalinity of calcium hydroxide in the calcium carbonate is neutralizedwith calcium bisulphite, preferably by the use of the reaction shown inmy application Ser. No. 561,241, namely, CB.(HSO3)2+Ca(OH)22CaSO3+2I-I2O a superior precipitated product is obtained.

By washin the precipitate with water, soluble matter is substantiallycompletely removed. The sodium sulphite that may be present in theprecipitate as the result of the above process may, if not washed out,be oxidized to sodium sulphate by the use of a small amount of oxidizingmaterial such as a solution of calcium hypochlorite, as set forth inapplication Serial No. 561,241.

The superiority of the product obtained in the above process is due tounusual light refractions resulting particularly from a mixture ofcalcium carbonate, crystalline calcium sulphite (CaSOaZHzO) andamorphous calcium sulphite.

One of the major items of expense in preparins Precipitated calciumcarbonate for satisfactory commercial use has for many years presenteditself in the necessary neutralization oi the stronger alkalineassociated substances. By the use of the reactions shown in myapplication Serial No. 561,241 I have found a method of neutralizationthat is inexpensive partly because sulphurous acid is readily producedfrom sulphur and partly because a solution of calcium bisulphite willproduce an extraordinary yield of over pounds of additional weight offiller for every 32 pounds of sulphur used. The reprecipitation of thecalcium sulphite from calcium bisulphite solution is readily effected bythe use of calcium hydroxide so that it may be seen that calciumcarbonate, calcium hydroxide and calcium bisulphite solutions may bemixed in proportions such that the resultant substance will be a mixtureof calcium carbonate and calcium sulphite in any desired orpredetermined ratio. The reaction at room temperature between a solutionof calcium bisulphite and calcium hydroxide or calcium carbonateproduces a calcium sulphite which is mainly non-crystalline. Microscopicexamination indicates a mere trace of the needle-like crystallinestructure surrounded by an amorphous or non-crystalline calciumsulphite. This characteristic structure lends itself to a highpercentage of retention of the filler and a high degree of opacity. I amable to control the ratio of crystallinecalcium sulphite to amorphouscalcium sulphite by controlling the reaction temperatures and also bycontrollin the amount of sulphite radical associated with the calciumbisulphite. I1 boiling temperatures are used, the product will belargely of crystalline structure. At temperatures around 20 C. theproduct will be mainly non-crystalline and without water ofcrystallization. The product may be identified by chemical analysis andmicroscopic examination.

By way of definition, it is noted that calcium sulphite in itscrystalline state includes two molecules of water of crystallization andin its amorphous state no water is chemically combined therewith.

The crystalline product is obtained from complete solution rather thanfrom equilibrium in decomposition. Crystalline calcium sulphite may bemade by a method described in my application Serial No. 561,241, namelyby boiling or driving on. the sulphur dioxide by reduced pressure fromcalcium bisulphite solution in sulphurous acid. A solution of calciumacid carbonate in carbonic acid upon boiling or driving off of carbondioxide by reduced pressure will give crystailine calcium carbonate. Thecrystalline material is improved, however, from the standpoints ofopacity and color, by the presence of amorphous calcium sulphite.

To obtain amorphous calcium sulphite, lime water or milk of lime isadded to a solution of calcium acid sulphite in sulphurous acid, thefollowing reaction taking place:

To obtain amorphous calcium carbonate, lime water or milk of lime isadded to a solution of calcium bicarbonate in carbonic acid to obtainthe following reaction:

Ca(HC03) 2+ (X) H2CO3+ (X+1) Ca(OI-I) 2- (X+2) CaCO3(amorphous) +2(X+ 1)H2O Another method of forming amorphous carbonate is by treating calciumcarbonate with carbonic acid as follows:-

(X) CaCO3+ (Y) HzCOc (X-Y) CaCOa( amorphous) (Y) CB.(HCO3) 2 In theabove equations, magnesium may be substituted for part of the calcium.

Amorphous, vesicular calcium sulphite is the result of treating calciumsulphite with sulphurous acid, in the following reaction:

(X) CB-SOa (Y) 132803 (X-Y) Casosiamorphous, vesicular) (Y) Ca(HSO3) 2Excellent results as a filler, may be obtained by mixing my product withother fillers such as calcium sulphate, crystalline or amorphous, orclay. These may both be added, if desired. A substance such as clay, ora substance having no regular crystalline shape may be mixed with myproduct particularly if it contains crystalline bodies to furtherimprove their opacity or covering power.

In producing amorphous calcium sulphite or crystalline calcium sulphiteor a mixture of the two, either chemically precipitated carbonate ormechanically pulverized carbonate may be used or sulphurous acid may beallowed to react on limestone. Chemically precipitated carbonate willgive asuperior product.

The following are examples of compositions prepared in accordance withthe present invention:

Example I Per cent 300 mesh ground lime stone 85 Amorphous calciumsulphite 8 Crystalline calcium sulphite 2 Clay Example II Crystallinecalcium sulphite 20 Amorphous calcium sulphite 40 English china clay 40Example III Calcium sulphite amorphous 74 Calcium sulphite crystalline18.6 Calcium sulphate 7.4

Example IV Per cent Calcium carbonate 50 Calcium sulphite amorphous 33Calcium sulphite crystalline 16% Example V Calcium carbonate 84Magnesium carbonate l3 Alkaline earth sulphites 3 While calciumcarbonate is used in the preparation of the product of the presentinvention, it has been found that dolomite, a natural carbonate ofcalcium and magnesium in varying proportions, or other magnesiumlimestones including magnesite are also suitable for such use. Theproduct resulting from the reaction of dolomite with calcium bisulphitesolution, for instance, contains a mixture of calcium and magnesiumsulphites which mixture may be employed in pigments, fillers, andloading materials of the type described. Magnesium sulphite alone mayalso be produced for use in the product described, from magnesiumcarbonate or hydrate. The carbonates may be reacted upon by thebisulphite solution to convert a given batch completely to sulphites, ora given batch of carbonates may be partially converted to form a mixtureof carbonate and sulphite material.

Depending upon the amount of calcium bisulphite or sulphurous acid addedto an alkaline carbonate material the resulting filler product mayeffect a neutral or slightly acid reaction when suspended in water or itmay effect a slightly alkaline reaction if desired.

In the treatment of carbonate or other suspensions with carbonic acid,by one method carbon dioxide gas is bubbled through the suspension in anaqueous medium. The carbon dioxide may be introduced in pure form or inthe form of waste gases such as combustion gases or lime kiln gases. Thebubbles are preferably finely divided and uniformly distributed.

Another method is to form a cake of the carbonate, or other material tobe treated, on a screen by differential pressure as in a plate or leaftype filter press and pass a solution of carbonic acid through the cake.

When using a dolomitic lime as the original raw material there may bedeveloped during the reaction with carbon dioxide a basic magnesiumcarbonate. All limes contain certain amounts of magnesium. I have foundit desirable in some cases to make certain that no basic magnesiumcarbonate be present. I can overcome this by treatment with acids. usingcarbonic, sulphurous or sulphuric acids for the treatment.

This basic carbonate may also be present with carbonic acid. In suchcase, the addition of a hydroxide such as calcium hydroxide will assistin forming the normal carbonate.

I have also found it desirable from the standpoint of opacity or hidingpower not to overcome the basic carbonate by recrystallization of themagnesium salts present. -These salts, particularly the basic carbonate,should be changed either to the normal carbonate or to soluble salts andwashed out as the perfect crystals of magnesium salts make the tillermore transparent or cause less opacity.

Mixtures of calcium and magnesium carbonate obtained by slaking a burneddolomitic limestone and treating the slaked material with carbondioxide, when treated with aluminum sulphate. sulphurous acid, calciumor magnesium bisulphites, or zinc or copper compounds, such ascarbonate, sulphates, chlorides ,nitrates, acetates and sulphites areconverted into finely divided prodnets of high quality and of unusualopacity and obscuring power. In certain instances it is not necessary tocompletely neutralize the alkalinity of the mixture of carbonates, adesirable product being prepared by partial neutralization of alkalinematter or by making the product slightly acid. The product obtained,when, for instance, sulphurous acid or bisulphites the preferred acidmaterials, are used for changing the alkalinity, comprises calcium andmagnesium sulphites, and calcium and magnesium carbonates in which themagnesium compounds may be present in substantially smaller amount thanis chemically equivalent to the calcium compounds present.

If desired, material may be added to further improve the opacity of afiller or pigment of the type including calcium sulphite, calciumcarbonate, magnesium sulphite, magnesium carbonate and the like. Forinstance, a zinc salt including zinc sulphite may be dissolved in any ofits solvents including sulphurous acid solution to form a solution whichwhen reacted with calcium carbonate or calcium hydroxide, magnesiumcarbonate or magnesium hydroxide will cause a precipitation of zinccompound the chemical structure of which is not definitely known. Thus aresultant mixture of zinc compound with calcium carbonate, calciumsuiphite, magnesium carbonate or magnesium sulphite, or withcombinations of these, may be formed.

One method is to dissolve scrap zinc in an acid, such as sulphuric acid,thereby forming a solution of zinc salt which will form a comparativelyinsoluble compound with the above-mentioned lime substances while at thesame time obtaining a resultant product of increased opacity or hidingpower over that obtained from the lime substance without the addition ofthe zinc salt. Less than by weight of the zinc salt may be combined withthe pigment to give good results.

Pigments or fillers may be treated to improve their color as well astheir opacity and sizability by the following treatments:

The solid particles are maintained in a dispersed condition in water.Any of various gelatinous substances or mordants including starch, glue,casein, aluminum hydroxide, silica gel, magnesium phosphate, or tungsticmaterials, or other substances which form lakes with coloring matter,are distributed on the suspended particles.

Mordants may be employed to the extent of under 10% of the weight of thefiller. Colorings are then added such as Prussian blue, inclanthrene,methylene blue, or other dyes.

Another treatment is to suspend the filler particles in water as setforth above in a mixing tank provided with stirrers. Then a reducingagent is introduced whereby the suspended particles are whitened. Forinstance, hydrogen sulphite gas may be bubbled through the mixture untilthe particles are whitened, or are somewhat paler than the originalparticles. Or about 1% or more of zinc sulphite may be mixed with thesuspension to obtain the whitening effect.

The above treatments as well as the aforegoing treatments with acidmaterials to improve opacity, may be used singly or combined with eachother and may be carried on in a tank provided with a stirring device.For instance, an

acid treatment, or a carbon dioxide treatment,

or a reduction treatment may be followed by a starch and colortreatment, or a reduction treatment may be followed by a treatment withan acidic reagent.

In the treatment of filler material withacidic' substances to improvethe opacity and sizability thereof by corroding the surfaces of thefiller particles, the acidic substances are added to suspensions of theparticles in such proportions as are insuillcient to completely convertthe substance of the filler into the salt of the acid used. Whencontacting the suspension of filler particles in an aqueous medium withcarbon dioxide or with sulphur dioxide, these gases are bubbled throughthe suspension until the particles become more opaque than the originalparticles, after which the passage of gases may be discontinued or thefiller may be separated from the suspending medium. In the use ofsolutions of acidic substances as reagents these may be added to thesuspensions in predetermined proportions. In the acid treatments, theproportions determine the degree of corrosion of the surfaces of thefiller particles and are limited only to the extent that the acid issumcient to eifect a surface change rather than a complete chemicalconversion.

The treatment of filler material set forth herein is applicable to thefiller material disclosed in my copending application Serial No.679,994, filed July 11, 1933, issued November 7, 1939 as Patent No.2,178,606, and the methods disclosed in the latter application forimproving the said filler material may be utilized in the treatment offillers disclosed herein.

Where in the claims I use the expression "reactive, inorganic,substantially water-soluble compound having a sulphite radical," I meansulphurous acid and/or blsulphite, either calcium, magnesium or othersubstantially watersoluble inorganic bisulphite.

In the various formulas given the proportions are expressed in terms ofweight. Various mixtures of the materials mentioned, in widely varyingproportions, may be prepared.

The product of the present invention finds its use in paper, paints,textiles, plastics, resinous materials, such as rubber and the like.

Patent No. 2,186,040. issued January 9, 1940. upon which thisapplication for reissue is based, is a division of my copendingapplication Serial No. 679,993, filed July 11, 1033.

What I claim is:

1. A process of preparing a pigment product, comprising treatingalkaline earth metal carbonate particles in the presence of water with areactive, inorganic, substantially water-soluble compound having asulphite radical to partially dissolve the alkaline earth metalcarbonate particles to form substantially water-insoluble, substantiallyopaque alkaline earth metal carbonate pigment particles having corrodedsurfaces.

2. A process of preparing a pigment product, comprising treatingalkaline earth metal carbonate particles in the presence of water with areactive, inorganic, substantially water-soluble compound having asulphite radical to partially dissolve the alkaline earth metalcarbonate particles to form substantially water-insoluble, substantiallyopaque alkaline earth metal carbonate pigment particles having corrodedsurfaces, and subsequently precipitating on the surface of the corrodedparticles alkaline earth metal sulphite.

3. A process of preparing a pigment product, comprising treating, in thepresence of water, particles of material comprising calcium carbonatewith a reactive, inorganic, substantially water-soluble compound havinga sulphite radical to partially dissolve said particles to thereby formsubstantially water-insoluble, substantially opaque pigment particlescomprising calcium carbonate, having corroded surfaces.

4. A process of preparing a pigment product, comprising treatingdolomitic limestone particles in the presence of water with a reactive,inorganic, substantially water-soluble compound having a sulphiteradical to partially dissolve said dolomitic limestone particles tothereby form substantially water insoluble, substantially opaquedolomitics limestone pigment particles having corroded surfaces.

5. A process of preparing a pigment product, comprising treating, in thepresence of water, particles of material comprising calcium carbonateand magnesium carbonate with a reactive, inorganic, substantiallywater-soluble compound having a sulphite radical to partially dissolvesaid particles to form substantially waterinsoluble, substantiallyopaque pigment particles of material comprising calcium carbonate andmagnesium compound, said particles having corroded surfaces.

6. A process of preparing a pigment product, comprising treating calciumcarbonate particles in the presence of water with sulphur dioxide tothereby partially dissolve said particles, and substantially separatingthe resulting particles from the resulting solution to obtainsubstantially water-insoluble, substantially opaque calcium carbonatepigment particles having corroded surfaces.

7. A process of preparing a pigment product, comprising treating, in thepresence of water, particles of material comprising calcium carbonatewith sulphur dioxide thereby partially dissolving said particles,substantially separating the resulting particles from the resultingsolution, and treating the resulting particles with a neutralizing agentto form substantiallywaterinsoluble, substantially opaque pigmentparticles of material comprising calcium carbonate, the surfaces ofwhich particles have been corroded.

8. A process of preparing a pigment product, comprising burningdolomitic limestone, slaking the resulting lime and magnesia to form thehydroxides, treating the same in the presence of water with sulphurousacid until a portion thereof is dissolved, and substantially separatingthe resulting substantially insoluble material from the resultingliquid, to form substantially waterinsoluble, substantially opaquepigment particles having corroded surfaces.

9. A process of preparing a pigment product, comprising burningdolomitic limestone, slaking the resultant lime and magnesia to form thehydroxides, treating the same in the presence of water with sulphurousacid until a. portion there" of is dissolved to produce corrodedparticle surfaces, and treating the resulting mixture with alkaline.earth metal hydroxide to precipitate dissolved alkaline earth metalmaterial in the form of alkaline earth metal sulphite on said particlesurfaces to form substantially water-insoluble substantially opaquepigment particles having coated surfaces.

10. A process of preparing a pigment product. comprising treating in thepresence of water a mixture comprising calcium carbonate and calciumhydroxide, obtained in the reaction be tween sodium carbonate and excesscalcium hydroxide, with calcium bisulphite solution to form calciumsulphite from part of the calcium carbonate and to convert the calciumhydroxide in the said mixture into calcium sulphite, to obtainsubstantially water-insoluble, substantially opaque pigment particleshaving coated surfaces.

11. A process of preparing a pigment product, comprising treating, inthe presence of water, particles comprising a mixture of calciumcarbonate and calcium hydroxide with a reactive, inorganic,substantially water-soluble compound having a sulphite radical topartially dissolve the calcium carbonate particles to form substantiallywater-insoluble. substantially opaque calcium carbonate pigmentparticles having corroded surfaces and to precipitate calcium sulphiteon the corroded surfaces.

'12. A process of preparing a pigment product, comprising treatingalkaline earth metal carbonate particles in the presence of water with areactive, inorganic, substantially water-soluble compound having asulphite radical to form substantially water-insoluble, substantiallyopaque alkaline earth metal carbonate pigment particles having surfacescoated with alkaline earth metal sulphite.

13. A method of producing a pigment product in an extremely finelydivided state which comprises first precipitating from a lime-bearingsuspension, by means of a reactive, inorganic, substantially watersoluble compound having a sulphite radical, particles of normal calciumsulphite, and then improving said particles for pigment purposes bycausing solution of only a portion of said normal calcium sulphite byconversion of the same into the bisulphite through reaction thereof withsulphurous acid.

14. A method of producing a pigment produc in an extremely finelydivided state which comprises first precipitating from a lime bearingsuspension, by means of a reactive, inorganic, substantially watersoluble compound having a sulphite radical, particles of normal calciumsulphite, then improving said particles for pigment purposes by causingsolution of only a portion of said normal calcium sulphite by conversionof the same into the bisulphite through reaction thereof with sulphurousacid, and then adding milk of lime to substantially neutralize saidbisulphite.

15. A method of producing a pigment product in an extremely finelydivided state which comprises first precipitating from a lime-bearingsuspension, by reaction with sulfurous acid, particles of normal calciumsulphite, improving said particles for pigment purposes by addingfurther sulfurous acid to effect only partial conversion and dissolutionof the sulphite, and separating the remaining particles from thesuspending medium.

16. A method of producing a pigment product sulphite in an extremelyfinely divided state which comprises first precipitating from alimebearing suspension, by reaction with sulfurous acid, particles ofnormal calcium sulfite, improving said particles for pigment purposes byaciding further sulfurous acid to effect only partial conversion anddissolution of the sulphite, and then adding sufficient milk of lime tosubstantially neutralize the suspension.

1'7. A method of producing a pigment product in a finely divided statewhich comprises first precipitating a normal salt from a lime-bearingsuspension by reacting therewith at least one of the group of acidicagents consisting of substances that will combine a sulphite radicalwith the lime and substances that will combine a carbonate radical withthe lime, and then improving said particles for pigment purposes bytreating the resulting suspension with a reactive, inorganic,substantially water-soluble compound comprising tree sulfurous acid tocause only partial dissolution of the precipitated normal salt.

18. A process oi preparing a pigment product in a finely divided statewhich comprises improving particles of the group consisting of calciumcarbonate and calcium sulflte particles for pigment purposes by treatingthe same in the presence of water with a reactive, inorganic,substantially water soluble compound comprising tree sulturous acid tocause only partial dissolution of said particles.

JOSEPH n PLUMST'EAD.

